Video surveillance cameras are known to experience periodic focus thrashing, or “focus hunt,” issues. Focus hunt is a condition wherein a camera continuously manipulates the focus for an extended period of time without successfully achieving a steady state condition. This is particularly common for outdoor cameras operating during nighttime hours. During such periods, cameras open their iris to allow as much light collection as possible; however, a side effect of a wide open iris is a severe reduction of a depth-of-field of the camera. Objects appearing at different focal lengths (particularly moving vehicular lights) cause the camera to thrash back and forth between two different points of focus.
For example, a focus hunting problem is described in a technical bulletin for a VG5 800 Series AutoDome camera (Technical Bulletin VG5 800 Series AutoDome CCTV, “Autofocus ‘Focus Hunting,’” Oct. 25, 2012). The bulletin describes that the VG5-825 series AutoDome camera may experience issues when attempting to focus on a scene, that is, the AutoDome camera may appear to be “focus hunting,” where the focus may move between the near and far focus limits. The bulletin further describes that there is no resolution for the autofocus issue that will completely eliminate the focus hunt issue and proposes that the user control focus hunting by pre-setting a time limit for how long the camera attempts to focus on a scene. Other known schemes for controlling a focus hunt issue include a user manipulating a camera position to achieve a steady focus state, for example, sending a pan-tilt-zoom (PTZ) command to re-adjust the camera's Field-of-View (FOV) to a view which doesn't trigger the focus hunt issue, adjusting a camera's iris control, enabling a camera's wide dynamic range feature, or disabling automatic focus.
Each such scheme of controlling the focus hunt issue, other than the pre-set time limit, requires that a user manually observe the occurrence of the focus hunt issue. Some large municipalities, such as the City of Chicago, are building large scale surveillance networks with tens of thousands of cameras connected to a central office. Only a small percentage of those cameras are monitored live. The cameras may be recorded for forensic purposes. Thus, while video captured from such cameras has to be of high quality, due to the size of the system any problems affecting video quality may be left undetected for long periods of time, well beyond a time when the problem initially occurs. During the time a surveillance camera is experiencing a focus hunt issue, the resulting video may be unusable for forensic purposes.
For example, FIG. 1 depicts a sequence of snapshots 102, 112, 122 from a video stream of a video camera experiencing a “focus hunting” condition. Time-wise, the snapshots occur in the order snapshot 102, snapshot 112, and then snapshot 122. In order to demonstrate the condition of focus hunting, an enlarged image 104, 114, 124 of a neon store sign is depicted with respect to each of snapshots 102, 112, and 122. As illustrated in FIG. 1, the image of the neon store sign is clearly readable when the camera is in focus (images 104 and 124) and becomes illegible when the camera goes out of focus (image 114). In the event of a focus hunt issue, the video stream may constantly and repeatedly shift between the in-focused image 104, 124 and the out-of-focus image 114, resulting in a video stream of reduced value for forensic purposes. It is easy to envision that such focus hunt issues can serve to obscure important forensic details, for example, a face or a license plate, from a video recording.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. Those skilled in the art will further recognize that references to specific implementation embodiments such as “circuitry” may equally be accomplished via replacement with software instruction executions either on general purpose computing apparatus (e.g., CPU) or specialized processing apparatus (e.g., DSP). It will also be understood that the terms and expressions used herein have the ordinary technical meaning as is accorded to such terms and expressions by persons skilled in the technical field as set forth above except where different specific meanings have otherwise been set forth herein.